1. Field of the Invention
The present invention relates to a misfire detecting apparatus for an internal combustion engine and, particularly, to a misfire detecting apparatus for determining a misfire based on a rotational speed parameter according to an engine rotational speed.
2. Description of the Related Art
When performing a misfire determination based on the rotational speed parameter according to the engine rotational speed, an incorrect misfire determination may result if the rotational speed parameter changes due to a factor other than a factor of engine combustion. For example, in a high rotational speed region, where the engine rotational speed exceeds 5000 rpm, a rotational speed change due to torsion of the crankshaft (crankshaft torsion) of the engine may become relatively large. Accordingly, an incorrect misfire determination may easily be made when such a determination is based on the rotational speed parameter.
Japanese Patent Laid-open No. H7-151009 (JP '009) discloses a method for eliminating the influence of crankshaft torsion. Specifically, a correction coefficient for eliminating the influence of crankshaft torsion is previously calculated according to the engine rotational speed and the load on the engine and is stored in a map. During a normal engine operation, the map is retrieved according to the rotational speed and the load to calculate the correction coefficient and correct the detected rotational speed parameter with the correction coefficient.
According to the method disclosed in JP '009, it is necessary to make a map having correction coefficients, which are previously and experimentally obtained, and manpower for making the map is required. Further, the rotational speed changing due to crankshaft torsion greatly changes depending on the ambient temperature. Therefore, in order to perform an accurate correction, it is necessary to perform the correction according to not only the engine rotational speed and the load on the engine but also the air temperature. It is very difficult to perform a perfect correction.